Guanidine compounds for removal of oxyanions from aqueous solutions and for carbon dioxide capture

ABSTRACT

Methods for removing oxyanions from water according to the following steps: (i) dissolving an oxyanion precipitating compound into the aqueous source to result in precipitation of an oxyanion salt of the oxyanion precipitating compound; and (ii) removing the oxyanion salt from the water containing the oxyanion to result in water substantially reduced in concentration of the oxyanion; wherein the oxyanion precipitating compound has the following composition: 
     
       
         
         
             
             
         
       
     
     wherein A is a ring-containing moiety and X m−  is an anionic species with a magnitude of charge m. The invention employs bis-iminoguanidinium compounds according to Formula (1a) as well as neutral precursor compounds according to Formula (1), which can be used for removing undesirable species from aqueous solutions or air, such as removal of sulfate from water and carbon dioxide from air.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims benefit of U.S. Provisional ApplicationNo. 62/422,138, filed on Nov. 15, 2016, U.S. Provisional Application No.62/459,118, filed on Feb. 15, 2017, U.S. Provisional Application No.61/422,142, filed on Nov. 15, 2016, and U.S. Provisional Application No.62/514,997, filed on Jun. 5, 2017, all of the contents of which areincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Prime Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention generally relates to compositions useful forremoving undesirable species from aqueous solutions or air, such asremoval of sulfate from water and carbon dioxide from air, and tomethods for using such materials in removing such species.

BACKGROUND OF THE INVENTION

Effective separation of highly hydrophilic anions (e.g., sulfate,selenate, chromate, phosphate) from competitive aqueous solutionsremains a major challenge, despite the tremendous progress in anionreceptor chemistry over the past decade. In the particular case ofsulfate, although a significant number of sulfate-binding receptors havebeen reported (e.g., I. Ravikumar et al., Chem. Soc. Rev., 41, 3077,2012), they have shown limited success in the substantial removal ofthis anion from water. A significant obstacle in the development ofanion receptors is often the multi-step synthesis required for theirassembly, which generally involves tedious purifications and the use oftoxic reagents and solvents.

The removal of sulfate from seawater, in particular, continues to be anongoing challenge. Seawater contains very high levels of sulfate (˜3,000mg/L), and seawater is used on a large scale in oil-field injectionoperations. During such operations, the sulfate in the seawater combineswith strontium and barium found in rock to form barium and strontiumsulfate scale. The precipitation of barium and strontium sulfates ishighly detrimental to the process, such as by clogging lines anddestroying production wells. The conventional technology for removingsulfate from seawater is by nanofiltration, which can reduce sulfatelevels to about 50 mg/mL. However, some drawbacks to this approach arethe remaining high sulfate levels, the need to pressurize the system to20-30 bars, which results in a significant expenditure in energy, andmembrane fouling. Other methods involve scale-removing chemicals, butthese are known to be difficult to use and very expensive, and are notvery effective against sulfate scales. Another technology, known as theMD-LPP process, yields sulfate-free seawater, but the process has thesignificant drawbacks of employing high pressures, pre-concentrating thesea water, and use of organic solvents.

An approach for aqueous anion separation that has proven particularlyeffective is selective anion crystallization with organic compoundsfunctionalized with hydrogen bonding groups (e.g., a) R. Custelcean,Curr. Opin. Solid State Mater. Sci. 2009, 13, 68; b) R. Custelcean,Chem. Soc. Rev. 2010, 39, 3675; c) R. Custelcean, Chem. Commun. 2013,49, 2173). This approach combines elements of anion receptor chemistryand crystal engineering, as it entails recognition of the targeted anionthrough complementary hydrogen bonding, and formation of stable crystalsthrough favorable packing. The challenge with anion crystallization fromwater is to identify anion-binding compounds that can effectivelycompete against the strong anion hydration, and that are also able toself-assemble with the anions of interest into crystals with low aqueoussolubility. In this respect, it has recently been discovered thatcrystallization of sulfate, in the form of extended [SO₄(H₂O)₅ ²⁻]_(n)clusters, with rigid and planar bis-guanidinium compounds, can strike afavorable energetic balance that allows for the efficient separation ofthe highly hydrophilic sulfate anion from water (R. Custelcean et al.,Angew. Chem. Int. Ed. 54, 10525, 2015; Angew. Chem. 127, 10671, 2015).In the foregoing prototype, the bis-guanidinium compound was synthesizedin situ by condensation of glyoxal with aminoguanidinium sulfate,resulting in a sulfate salt with low aqueous solubility(K_(sp)=3.2×10⁻⁷), comparable with that of SrSO₄. Although thesolubility of the foregoing bis-guanidinium sulfate salt is much lowerthan many other organic sulfate salts, the solubility remainsunacceptably high, particularly for use in oil-field injectionoperations involving competitive aqueous solutions of high ionicstrength, such as seawater. There would be a significant benefit in astraight-forward and cost-efficient process that could removesubstantially all sulfate from seawater without the use of pressure,nanofiltration, pre-concentration, and organic solvents.

SUMMARY OF THE INVENTION

In one aspect, the instant disclosure describes a process for removingan oxyanion (e.g., sulfate) from a aqueous source by contacting theaqueous source with specialized bis-iminoguanidinium compounds that forma highly insoluble salt of the oxyanion, thereby precipitating asubstantial amount or substantially all of the oxyanion in the water. Aparticularly special aspect of the specialized bis-iminoguanidiniumcompounds described herein is the presence of a central ring-containingportion, such as a benzene or pyridine ring. By removal of the oxyanionsalt, such as by filtration, the oxyanion from the water can be easilyremoved. The bis-iminoguanidinium compounds described herein canadvantageously remove one or more oxyanions, such as sulfate, nitrate,chromate, selenate, phosphate, arsenate, carbonate, or bicarbonate,selectively or non-selectively while in the presence of other anionicspecies. The bis-iminoguanidinium compounds described herein can alsoadvantageously be recycled and re-used in the oxyanion removal process.The process described herein is advantageously straight-forward andcost-efficient while at the same time removing a substantial amount orsubstantially all of the oxyanion from seawater or other aqueous sourcewithout requiring pressure, nanofiltration, pre-concentration, andorganic solvents. The process described herein operates by simpleself-assembly of the compounds and oxyanion, thereby circumventing theneed for elaborate syntheses of compounds that precipitate the oxyaniondirectly without the aid of self-assembly.

In particular embodiments, the method for removing oxyanion from waterinvolves the following steps: (i) dissolving an oxyanion precipitatingcompound into the aqueous source to result in precipitation of anoxyanion salt of the oxyanion precipitating compound; and (ii) removingthe oxyanion salt from said water containing the oxyanion to result inwater substantially reduced in concentration of the oxyanion; whereinthe oxyanion precipitating compound has the following composition:

In the above Formula (1a), A is a ring-containing moiety; X^(m−) is ananionic species with a magnitude of charge m, where m is an integer ofat least 1, provided that X^(m−) is an anionic species exchangeable withthe oxyanion in the aqueous source before the oxyanion precipitatingcompound contacts the oxyanion in step (i), and X^(m−) is the oxyanionin the resulting oxyanion salt formed in step (i) and as separated fromthe water in step (ii). The subscript n is an integer of at least 1,provided that n×m=2. Moreover, one or more of the hydrogen atoms inFormula (1a), whether the hydrogen atoms are shown or not shown, may bereplaced with one or more methyl groups, respectively.

In another aspect, the instant disclosure describes a process forremoving carbon dioxide from a gaseous source by contacting the gaseoussource with an aqueous solution containing a specializedbis-iminoguanidine compound that forms a highly insoluble salt of carbondioxide in the form of carbonate or bicarbonate. The bis-iminoguanidinecompounds described herein are analogous to the structure shown inFormula (1a), except that the bis-iminoguanidine compounds are neutralbefore dissolution into the aqueous solution. The structure of theneutral bis-iminoguanidine analogue is provided as follows (where A is aring-containing moiety, as described above under Formula (1a)):

Once dissolved in aqueous solution, the bis-iminoguanidine neutralcompounds react with water to form a bis-guanidinium-dihydroxidespecies, which corresponds to Formula (1a) when X is hydroxide (HO⁻),i.e., with n=2 and m=1. The bis-guanidinium-dihydroxide species reactswith dissolved carbon dioxide that has been converted to carbonate orbicarbonate in the aqueous solution to form a carbonate or bicarbonatesalt of the bis-iminoguanidinium compound shown in Formula (1a). In someembodiments, a liquid sorbent (e.g., a hydroxide- or amine-containingbased) is first used for absorbing carbon dioxide from the gaseoussource and converting the carbon dioxide to carbonate or bicarbonate.The sorbent containing the carbonate or bicarbonate is then contactedwith the bis-guanidinium-dihydroxide species in aqueous solution to forma carbonate or bicarbonate salt with the bis-guanidinium species. Bywhichever process is used, the carbonate or bicarbonate salt of thebis-iminoguanidinium species can then be easily removed from thesolution, such as by filtration. The bis-iminoguanidine compoundsdescribed herein can also advantageously be recycled and re-used in thecarbon dioxide removal process. Moreover, one or more of the hydrogenatoms in Formula (1), whether the hydrogen atoms are shown or not shown,may be replaced with one or more methyl groups, respectively.

In particular embodiments, the method for removing carbon dioxide from agaseous source involves the following steps: (i) contacting the gaseoussource with an aqueous solution containing a carbon dioxide complexingcompound to result in precipitation of a carbonate or bicarbonate saltof the carbon dioxide complexing compound; and (ii) removing thecarbonate or bicarbonate salt from the aqueous solution. In the aqueoussolution, before contact with dissolved carbon dioxide, the carbondioxide complexing compound has the bis-guanidinium-dihydroxidecomposition described above, within the scope of Formula (1a). In someembodiments, the bis-guanidinium-dihydroxide composition is directlyadded to the aqueous solution before or during contact of the aqueoussolution with the gaseous source. In other embodiments, thebis-guanidinium-dihydroxide composition is produced in situ bydissolving the neutral bis-iminoguanidine analogue according to Formula(1) into the aqueous solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D. X-ray crystal structure views of1,4-benzene-bis(iminoguanidinium) sulfate salt (BBIG-SO₄). FIG. 1A:ORTEP representation showing the planar BBIG cation and the sulfate withthe two water molecules of hydration. FIG. 1B: [(SO₄)₂(H₂O)₄]⁴⁻ cluster.FIG. 1C: Stacking of the BBIG cations, with the dashed linescorresponding to the C═N(imine) . . . Ph and H₂N . . . C═N(imine)intermolecular contacts.

FIG. 1D: Hydrogen bonding of the sulfate-water clusters by theguanidinium groups of the BBIG stacks, viewed down the crystallographica axis.

FIGS. 2A-2C. X-ray crystal structure views of1,4-benzene-bis(iminoguanidinium) nitrate salt (BBIG-NO₃). FIG. 2A:ORTEP representation. FIG. 2B: Stacking of the BBIG cations. FIG. 2C:Hydrogen bonding of the nitrate anions by the guanidinium groups of theBBIG cations.

FIG. 3. Van't Hoff plot for dissolution of BBIG-SO₄ in the 15-358° C.temperature range.

FIG. 4. Schematic diagram showing a complete separation cycle forsulfate removal by crystallization of BBIG-SO₄. Step A: In situsynthesis of BBIG dichloride salt from aqueous aminoguanidinium chlorideand terephthaldehyde; Step B: Selective crystallization of BBIG-SO₄;Step C: Filtration of BBIG-SO₄; Step D: Compound recovery byneutralization of BBIG-SO₄ with NaOH and crystallization of neutralBBIG; sulfate is removed as aqueous Na₂SO₄; Step E: Regeneration of theBBIG dichloride salt, which can be recycled for another separationcycle.

FIG. 5. Schematic diagram showing a CaSO₄ dissolution cycle using theBBIG compound, starting with BBIG-Cl.

FIG. 6. Schematic diagram showing CO₂ capture from ambient air withaqueous PyBIG, leading to crystallization of PyBIGH₂(CO₃)(H₂O)₄(single-crystal neutron structure shown). The CO₂ is released, and thePyBIG compound is regenerated quantitatively by relatively mild heatingof the carbonate crystals.

FIGS. 7A, 7B. FIG. 7A: General schematic showing a direct air capture(DAC) cycle combining CO₂ absorption by an aqueous sorbent,crystallization of PyBIGH₂(CO₃)(H₂O)₄ and sorbent regeneration, and CO₂release and PyBIG regeneration by heating of the carbonate crystals.FIG. 7B: Schematic diagram showing the overall CO₂ separation cycle inwhich atmospheric CO₂ capture using PyBIG is combined with CO₂ sorptionby an alkali carbonate in solution.

FIGS. 8A, 8B. Chart showing absorption of atmospheric CO₂ into 1 Maqueous solutions of glycine/KOH (FIG. 8A) and sarcosine/KOH (FIG. 8B)as a function of time. Squares and dots correspond to carbonate andtotal CO₂ (carbonate+carbamate) loadings, respectively.

FIG. 9. Decrease in the total CO₂ concentration of the loaded glycine(dots) and sarcosine (squares) sorbents during regeneration with PyBIG.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the invention is directed to specializedbis-iminoguanidine or bis-iminoguanidinium (i.e., “BIG”) compoundshaving a central ring-containing moiety (A) attached to twoiminoguanidine or iminoguanidinium groups. The compounds are within thescope of the following generic structure:

Although Formula (1) depicts a specific tautomeric arrangement, Formula(1) is intended to include any other tautomers that can be derived fromor interconvert with the tautomer shown in Formula (1). As well known,tautomeric structures have the same atomic connections (aside from oneor more protons) but differ in the placement of double bonds, generallywith concomitant relocation of one or more protons. Some examples oftautomers of Formula (1) are provided as follows:

Formula (1) is also intended to include any regioisomers that may differin the connection points of the two aminoguanidine or aminoguanidiniumgroups on the ring-containing moiety (A). Thus, as an example, if A istaken as a benzene (phenylene) ring, the two shown aminoguanidine oraminoguanidinium groups may be located at the 1,4 (para), 1,3 (meta) or1,2 (ortho) positions. In some embodiments, the aminoguanidine oraminoguanidinium groups are located the farthest from each other on thering-containing moiety. In the case of a benzene ring, the farthestpositions correspond to the 1,4 (para) positions.

In the event that the structure according to Formula (1) possesses oneor more stereocenters, Formula (1) is intended to include all resultingstereoisomers. The stereoisomer may include one or more enantiomersand/or diastereomers.

Moreover, although Formula (1) depicts a neutral molecule, Formula (1)is intended to encompass salt forms of the Formula (1). The salt formsgenerally correspond to those that can be produced by reaction of theneutral form of Formula (1) with a mineral acid or alkyl halide, whichresults in protonation or alkylation of one or more of the shown amineor imine groups. The salt form of Formula (1) can be expressed by thefollowing sub-generic structure:

As the structure in Formula (1a) is within the scope of Formula (1), itis understood that Formula (1a), like Formula (1), includes all possibletautomers, regioisomers, and stereoisomers described above for Formula(1). Thus, the positive charge shown in Formula (1a) may be located onany of the other nitrogen atoms through tautomerizaton. As well known inthe case of tautomers, the positive charge is generally distributedamong all atoms capable of holding a positive charge in the varioustautomers. Likewise, it is well known that partial double bond characteris generally present among all of the bonds capable of engaging indouble bonds in the various tautomers. Moreover, the structures inFormulas (1) and (1a) both include the possibility of one or more of thehydrogen atoms in Formula (1) or (1a), whether the hydrogen atoms areshown or not shown in the formula, being replaced with one or moremethyl groups, respectively.

In Formula (1a), X^(m−) is an anionic species with a magnitude of chargem, where m is an integer of at least 1, and n is an integer of at least1, provided that n×m=2. The anionic species may be any anionic speciesthat, when complexed as a salt with the bis-aminoguanidinium portionshown in Formula (1a), can be exchanged for another anionic speciesdesired to be removed from an aqueous solution. As the different anionicspecies have different dissociation constants, any anionic species maybe useful in exchanging with another anionic species to be removed froman aqueous source. The anionic species may also represent a species thathas been removed from an aqueous solution, wherein the resulting salt ofthe removed anion and bis-aminoguanidinium portion shown in Formula (1a)is valuable as a precursor for producing a neutral form of Formula (1a)or by exchanging with another anionic species that can be used toexchange with and remove another anionic species of interest. Theanionic species (X^(m−)) can be, for example, a halide, such asfluoride, chloride, bromide, or iodide. The anionic species canalternatively be a halide equivalent (or pseudohalide), such asmethanesulfonate (mesylate), trifluoromethanesulfonate (triflate),tosylate, cyanate, thiocyanate, cyanide, or a sulfonamide anion, such asbis(trifluoromethane)sulfonamide (i.e., bistriflimide). The anionicspecies may alternatively be a borate anion, such as tetrafluoroborate,tetrakis(pentafluorophenyl)borate, ortetrakis[3,5-bis(trifluoromethyl)phenyl]borate. The anionic species mayalternatively be hexafluorophosphate (PF₆ ⁻). The anionic species mayalternatively be hydroxide, or an alkoxide (e.g., methoxide orethoxide). The anionic species may alternatively be a carboxylatespecies, such as formate, acetate, propionate, or glycolate. In otherembodiments, the anionic species (X^(m−)) can be an oxyanion. As usedherein, the term “oxyanion” refers to an anion having at least three orfour oxygen atoms, wherein the oxygen atoms are generally all bound to acentral element. Some examples of oxyanions include sulfate (e.g., SO₄²⁻), nitrate (NO₃ ⁻), chromate (e.g., CrO₄ ²⁻), selenate (e.g., SeO₄²⁻), phosphate (e.g., PO₄ ³⁻), arsenate (AsO₄ ³⁻), carbonate (CO₃ ²⁻),bicarbonate (HCO₃ ⁻), and perchlorate (ClO₄ ⁻). The oxyanions providedabove may or may not also include related derivatives. For example,unless otherwise stated, the term “sulfate” may also include thiosulfate(S₂O₃ ²⁻), bisulfate (HSO₄ ⁻), and sulfite (SO₃ ²⁻). Similarly, the term“chromate” may also include Cr₂O₇ ²⁻ (dichromate). Similarly, the term“phosphate” may also include hydrogenphosphate (HPO₄ ²⁻),dihydrogenphosphate (H₂PO₄ ⁻), pyrophosphate (P₂O₇ ⁴⁻), thiosphosphates(e.g., PO₃S³⁻ or PO₂S₂ ³⁻), and phosphite (e.g., PO₃ ³⁻, HPO₃ ²⁻, orH₂PO₃ ⁻). The oxyanion may also be selected from among less commonspecies, such as tungstate, vanadate, molybdate, tellurate, andstannate.

The ring-containing moiety (A) is or includes any cyclic group thatincludes at least one, two, three, or four carbon ring atoms. Since thecyclic group is attached to two iminoguanidine or iminoguanidiniumgroups, the cyclic group in the ring-containing moiety (A) necessarilyincludes two sites engaged in bonds, either directly, or indirectly viaa linker, to the iminoguanidine or iminoguanidinium groups. Typically,the two sites in the ring (A) linked, directly or indirectly, to theiminoguanidine or iminoguanidinium groups are ring carbon atoms. In someembodiments, the ring-containing moiety is or includes a monocyclicring, i.e., a single ring not bound or fused to another ring. In otherembodiments, the ring-containing moiety is or includes a ring system,wherein the term “ring system” refers to a polycyclic moiety (e.g., abicyclic or tricyclic moiety). The cyclic group can be polycyclic byeither possessing a bond between at least two rings or a shared (i.e.,fused) bond between at least two rings. The one or more rings in thering-containing moiety is typically a five-membered, six-membered, orseven-membered ring.

In one set of embodiments, the ring-containing moiety (A) is or includesa carbocyclic ring or ring system. The term “carbocyclic” indicates thatthe ring or ring system contains only carbon ring atoms. The carbocyclicring or ring system can be saturated or unsaturated. Some examples ofcarbocyclic rings that are monocyclic and saturated include cyclopentyl,cyclohexyl, and cycloheptyl rings. Some examples of carbocyclic ringsthat are monocyclic and unsaturated (which may be aliphatic or aromatic)include cyclopentenyl, cyclopentadienyl, cyclohexenyl, cyclohexadienyl,cycloheptenyl, cycloheptadienyl, and phenylene (benzene) rings. Someexamples of carbocyclic rings that are polycyclic and saturated includedecalin, norbornane, bicyclohexane, and 1,2-dicyclohexylethane ringsystems. Some examples of carbocyclic rings that are polycyclic andunsaturated include naphthalene, anthracene, phenanthrene, phenalene,and indene ring systems.

In another set of embodiments, the ring-containing moiety (A) is orincludes a heterocyclic ring or ring system. The term “heterocyclic”indicates that the ring or ring system contains at least one ringheteroatom. The ring heteroatom is typically selected from nitrogen,oxygen, and sulfur. The heterocyclic ring or ring system can besaturated or unsaturated. Some examples of heterocyclic saturated ringsor ring systems include those containing at least one ring nitrogen atom(e.g., pyrrolidine, piperidine, piperazine, imidazolidine, azepane, anddecahydroquinoline rings); those containing at least one ring oxygenatom (e.g., oxetane, tetrahydrofuran, tetrahydropyran, 1,4-dioxane,1,3-dioxane, and 1,3-dioxepane rings); those containing at least onering sulfur atom (e.g., tetrahydrothiophene, tetrahydrothiopyran,1,4-dithiane, 1,3-dithiane, and 1,3-dithiolane rings); those containingat least one ring oxygen atom and at least one ring nitrogen atom (e.g.,morpholine and oxazolidine rings); and those containing at least onering nitrogen atom and at least one ring sulfur atom (e.g., thiazolidineand thiamorpholine rings). Some examples of heterocyclic unsaturatedrings or ring systems include those containing at least one ringnitrogen atom (e.g., pyrrole, imidazole, pyrazole, pyridine, pyrazine,pyrimidine, 1,3,5-triazine, azepine, diazepine, indole, purine,benzimidazole, indazole, 2,2′-bipyridine, quinoline, isoquinoline,phenanthroline, 1,4,5,6-tetrahydropyrimidine,1,2,3,6-tetrahydropyridine, 1,2,3,4-tetrahydroquinoline, quinoxaline,quinazoline, pyridazine, cinnoline, and 1,8-naphthyridine rings); thosecontaining at least one ring oxygen atom (e.g., furan, pyran,1,4-dioxin, benzofuran, dibenzofuran, and dibenzodioxin); thosecontaining at least one ring sulfur atom (e.g., thiophene,thianaphthene, benzothiophene, thiochroman, and thiochromene rings);those containing at least one ring oxygen atom and at least one ringnitrogen atom (e.g., oxazole, isoxazole, benzoxazole, benzisoxazole,oxazoline, 1,2,5-oxadiazole (furazan), and 1,3,4-oxadiazole rings); andthose containing at least one ring nitrogen atom and at least one ringsulfur atom (e.g., thiazole, isothiazole, benzothiazole,benzoisothiazole, thiazoline, and 1,3,4-thiadiazole rings).

Some examples of compounds according to Formula (1a) include thefollowing:

Any of the above exemplary compounds may also be converted to therespective neutral analogue according to Formula (1) by removal of thetwo protons located on positively charged amine groups. Moreover, in anyof the above exemplary formulas, a hydrogen atom on a ring nitrogen atommay be replaced with a hydrocarbon group, such as a methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, phenyl, orbenzyl group. As also provided above, one or more of the hydrogen atomsin any of the above exemplary structures, whether the hydrogen atoms areshown or not shown, may be replaced with one or more methyl groups,respectively.

The compounds according to Formulas (1) and (1a) can be synthesized bymethods well known in the art. In particular embodiments, the compoundsaccording to Formulas (1) and (1a) are synthesized by reactingaminoguanidine or aminoguanidinium chloride (or a methylated derivativethereof) with a ring-containing dialdehyde or diketone under conditionswhere an imine linkage is formed between an amino group on theaminoguanidine or aminoguanidinium molecule and the carbon of thealdehyde or ketone group. The ring-containing dialdehyde or diketoneincludes a ring-containing moiety (A), as described above. A generalschematic of the process is provided as follows:

In the above scheme, A is a ring-containing moiety, as described above.The group R is typically hydrogen (which corresponds to a dialdehydereactant), but R may be a hydrocarbon group, such as methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, phenyl, orbenzyl group, which corresponds to a diketone reactant. The above schemeis meant to be inclusive of producing a neutral bis-iminoguanidinecompound according to Formula (1). To produce the neutral compound,aminoguanidine can be used in the above scheme instead of a guanidiniumsalt, or alternatively, the guanidinium salt can be produced andconverted to the neutral guanidine compound by reaction with a base.Moreover, any one or more hydrogen atoms of the aminoguanidine oraminoguanidinium reactant may be replaced with one or more methylgroups, respectively, except that the aminoguanidine or aminoguanidiniumreactant should retain at least one primary amine group for reactionwith the dialdehyde or diketone. Alternatively, one or more hydrogenatoms of the bis-iminoguanidinium or bis-iminoguanidine product may bereplaced with one or more methyl groups by, for example, reaction withmethyl iodide.

In another aspect, the invention is directed to a method for removingone or more oxyanions from an aqueous source containing the oxyanion bycontact of the aqueous source with any of the bis-iminoguanidiniumcompounds (“oxyanion precipitating compounds”) of Formula (1a), asdescribed above, wherein the anionic species (X^(m−)) in Formula (1a),before contact with the aqueous source, is exchangeable with theoxyanion to be removed from the aqueous source. In some embodiments, theanionic species in Formula (1a), before contact with the aqueous source,is more specifically a halide or pseudohalide. The oxyanion(s) in theaqueous source can be one or more of any of the oxyanions describedabove. The aqueous source can be any source containing one or moreoxyanions to be removed. The oxyanion to be removed is generally presentin the aqueous source as an inorganic salt that is dissolved orsuspended in the aqueous source. In some cases, at least one of theoxyanions is in the form of an insoluble scale, such as CaSO₄, SrSO₄, orBaSO₄ scale. Scale is often a major problem in oil field injectionoperations and the method described herein offers a solution to scaleremoval. The aqueous medium is typically composed predominantly orcompletely of water, such as found in seawater, water from sewagetreatment, or aqueous effluent from an industrial or commercial process.However, in some embodiments, the aqueous medium may include an organicsolvent miscible in water, such as an alcohol, acetone, or the like.

In the method for removing one or more oxyanions from an aqueous source,the oxyanion precipitating compound is dissolved in the aqueous source.The foregoing dissolution process can be referred to as “step (i)”. Theoxyanion precipitating compound refers to any of thebis-iminoguanidinium compounds of Formula (1a) where the anionic species(X^(m−)) is exchangeable with the oxyanion to be removed from theaqueous source. That is, the anionic species (X^(m−)) in thebis-iminoguanidinium compound of Formula (1a), before being contactedwith and dissolved into the aqueous source, should be capable of beingreplaced with the oxyanion to be removed from the aqueous source. Forexample, the bis-iminoguanidinium compound of Formula (1a) may take theanionic species (X^(m−)) as a halide or pseudohalide, before contact ofthe bis-iminoguanidinium compound of Formula (1a) with the aqueoussource, in a situation where the oxyanion to be removed from the aqueoussource is sulfate, nitrate, chromate, selenate, phosphate, arsenate,carbonate, bicarbonate, or perchlorate. The oxyanion precipitatingcompound can be dissolved by any suitable means, such as by directlyadding solid oxyanion precipitating compound to the aqueous source or byadding a pre-made solution, suspension, or slurry of the oxyanionprecipitating compound to the aqueous source.

The oxyanion precipitating compound is added to the aqueous source insuch amount and under such conditions (e.g., temperature) that result inprecipitation of an oxyanion salt of the oxyanion precipitatingcompound. For example, in the case where the oxyanion precipitatingcompound being added to the aqueous source corresponds to thebis-iminoguanidinium compound of Formula (1a) where the anionic species(X^(m−)) is halide or a pseudohalide, and the oxyanion to be removedfrom the aqueous source is sulfate, the oxyanion precipitating compoundshould be added to the aqueous source in sufficient amount and underappropriate conditions to result in replacement of the halide orpseudohalide with sulfate in the bis-iminoguanidinium compound ofFormula (1a). The result is that the resulting precipitated saltcorresponds to a salt of the bis-iminoguanidinium compound of Formula(1a) where the anionic species X^(m−) is the oxyanion (e.g., sulfate)being removed from the aqueous source. Generally, the oxyanionprecipitating compound is added to the aqueous source in an amountcorresponding to at least, and generally above, the molar amount ofoxyanion expected to be contained within a sample of aqueous source tobe processed. The term “precipitation,” as used herein, refers to theseparation of the oxyanion salt, as a solid, from the aqueous source.The precipitate can be, for example, an amorphous solid (e.g., as scale,sludge, or powder) or crystalline material. In preferred embodiments,the precipitate is in crystalline form, since crystal formationfunctions as an additional driving force for removal of the oxyanionsalt from solution.

Following the dissolution of the oxyanion precipitating compound ofFormula (1a) and precipitation of the oxyanion salt of Formula (1a) instep (i), the precipitated oxyanion salt is removed from the aqueoussource to result in water substantially reduced in the concentration ofthe oxyanion originally present in the aqueous source. The removal stepcan be referred to as step (ii). The precipitated oxyanion salt can beremoved by any of the means well known in the art for removing solidmaterial from a liquid. The precipitated oxyanion salt can be removedby, for example, filtration, or by centrifugation followed by decanting,or by a combination thereof. By use of the oxyanion precipitatingcompounds described herein, the oxyanion salt being removed can bereduced by at least or above 98%, 99%, 99.5%, or 99.9% compared to theoriginal concentration of the oxyanion in the aqueous source.

In the process described above for removing one or more oxyanions froman aqueous source, the resulting precipitated oxyanion salt can beconveniently processed to regenerate the starting oxyanion precipitatingcompound according to Formula (1a). By regenerating the startingoxyanion precipitating compound, the process can advantageously includea recycling step, which makes the process further cost effective withminimal environmental impact. To regenerate the starting oxyanionprecipitating compound, the precipitated oxyanion salt (e.g., Formula(1a) in which X^(m−) is sulfate) can be reacted with a base (e.g., ametal hydroxide, organic amine, or ammonia) that converts the cationicform of the oxyanion precipitating compound (according to Formula (1a))to the neutral form depicted in Formula (1) while at the same timeforming a byproduct salt (e.g., metal sulfate, organoammonium sulfate,or ammonium sulfate, respectively) with the oxyanion originally boundwith the oxyanion precipitating compound. The neutral compound depictedin Formula (1) is then reacted with a protic acid (e.g., HCl, HBr, orHNO₃, etc.) to produce the original cationic form according to Formula(1a) with X^(m−) being the conjugate base of the acid used (e.g., Cl⁻,Br⁻, or NO₃ ⁻, respectively).

In another aspect, the invention is directed to a method for removingcarbon dioxide from a gaseous source by contacting the gaseous sourcewith an aqueous solution containing a bis-iminoguanidinium compound ofFormula (1a) wherein the anionic species (X^(m−)) is hydroxide. Byvirtue of the hydroxide anion in the bis-iminoguanidinium compound ofFormula (1a), the bis-iminoguanidinium compound of Formula (1a)functions as a carbon dioxide complexing (capturing) compound. Morespecifically, the hydroxide anion in the bis-iminoguanidinium compoundof Formula (1a) reacts with carbon dioxide to form a carbonate orbicarbonate anion. The resulting carbonate or bicarbonate anionassociates with the bis-iminoguanidinium compound of Formula (1a) toform a carbonate or bicarbonate salt of the bis-iminoguanidiniumcompound of Formula (1a). The gaseous source can be any volume of gascontaining carbon dioxide. The gaseous source can be, for example, air,waste gas from an industrial or commercial process, flue gas from apower plant, exhaust from an engine, or sewage or landfill gas.

In one embodiment, the bis-iminoguanidinium-hydroxide compound isprepared before it is dissolved in an aqueous medium to produce theaqueous solution. In another embodiment, thebis-iminoguanidinium-hydroxide compound is produced in the aqueousmedium in situ by adding a neutral bis-iminoguanidine compound accordingto Formula (1) to the aqueous medium, in which case the neutralbis-iminoguanidine compound spontaneously reacts, via its substantialalkalinity, with water molecules to form a bis-iminoguanidiniumaccording to Formula (1a) in which the anionic species is hydroxide.

In the carbon dioxide removal process, the gaseous source is contactedwith the aqueous solution containing the carbon dioxide complexingcompound by any means that permits the gaseous source to dissolve intothe aqueous solution. The gaseous source can, for example, be bubbledthrough the aqueous solution, with or without agitation of the aqueoussolution. Alternatively, the gaseous source may be sprayed or mistedwith the aqueous solution, which may be performed in the presence of alayer of the aqueous solution under agitation to further absorb thecarbon dioxide. When the carbon dioxide complexing compound contacts thedissolved carbon dioxide, the hydroxide anion in the carbon dioxidecomplexing compound reacts with the carbon dioxide so as to form acarbonate or bicarbonate salt of the carbon dioxide complexing compound,i.e., a carbonate or bicarbonate salt of the bis-iminoguanidiniumcompound of Formula (1a). The carbonate or bicarbonate salt of thebis-iminoguanidinium compound of Formula (1a) precipitates from theaqueous solution, either as amorphous powder form or in crystallineform, as described above in the oxyanion removal process. The foregoingcontacting and precipitating stage may be referred to as step (i).

After the carbonate or bicarbonate salt of the bis-iminoguanidiniumcompound of Formula (1a) is precipitated, the precipitate is removedfrom the aqueous solution by any suitable means, such as filtration, asdescribed above for the oxyanion removal process. The foregoing removalstep may be referred to as step (ii). Moreover, the resulting carbonateor bicarbonate salt can be processed, such as by heating, to recover thestarting bis-iminoguanidinium-hydroxide within the scope of Formula (1a)or the starting neutral bis-iminoguanidine compound within the scope ofFormula (1) with simultaneous evolution of carbon dioxide gas. Theevolved carbon dioxide gas may be stored and/or pressurized, asappropriate, and may be subsequently further processed or reacted in anindustrial or commercial process.

In the above-described process for capturing carbon dioxide, thebis-iminoguanidinium compound according to Formula (1a) captures thecarbon dioxide directly by converting carbon dioxide to carbonate orbicarbonate and forming a salt with the carbonate or bicarbonate.However, in some embodiments, the carbon dioxide to carbonate conversionprocess can be separated from the salt formation process. A separatedprocess can be particularly advantageous when using a CO₂ sorbent thatcan capture and convert the carbon dioxide to carbonate significantlyfaster than the bis-iminoguanidinium-hydroxide compound in Formula (1a).In this way, a highly efficient CO₂ sorbent can quickly capture andconvert carbon dioxide to carbonate, and a bis-iminoguanidinium compoundaccording to Formula (1a) can quickly form an insoluble salt with thecarbonate, thereby removing the carbon dioxide in a more efficientmanner. In the two-part process, the anion of the bis-iminoguanidiniumcan be any anion that can be replaced with carbonate or bicarbonate,such as hydroxide or a halide. The CO₂ sorbent is typically an aqueoussolution in which a base reactive with carbon dioxide is included. Thebase may be, for example, a metal hydroxide (e.g., NaOH or KOH), alkalicarbonate, or an amine-containing molecule other than those described inFormulas (1) and (1a), e.g., methylamine, ethylamine, ethylenediamine,ethanolamine, and amino acids (e.g., glycine, sarcosine, taurine,alanine, valine, leucine, serine, threonine, glutamine, asparagine,lysine, arginine, and phenylalanine).

Examples have been set forth below for the purpose of illustration andto describe certain specific embodiments of the invention. However, thescope of this invention is not to be in any way limited by the examplesset forth herein.

Examples Synthesis of 1,4-benzene-bis(iminoguanidinium) (BBIG) compounds

The synthetic process used in preparing the BBIG compounds is summarizedby the following scheme:

In brief, aqueous condensation of aminoguanidinium chloride withterephthalaldehyde led to the in situ formation of the1,4-benzene-bis(iminoguanidinium) cation (BBIG), which crystallized asthe sulfate (BBIG-SO₄) or nitrate (BBIG-NO₃) salt in the presence ofNa₂SO₄ or NaNO₃, respectively.

Preparation of BBIG-Cl (1):

4 grams of terephthalaldehyde and 7.26 grams of aminoguanidiniumchloride were added to 20 mL of ethanol in a 50 mL round bottom flaskequipped with a magnetic stir bar. The solution was heated to 60° C.using a hotplate, and stirred with a magnetic stir bar for 2 hours. Thesolution was cooled to 20° C. and allowed to sit for three hours, beforecollecting the solid by vacuum filtration through a filter-paperequipped Büchner funnel. The obtained solid was suspended in 20 mL ofethanol and the suspension heated on a hotplate until boiling. If thesolid did not go completely into solution at this point, one or moresmall aliquots (1 mL) of ethanol were added, followed by bringing thesolution to boiling until all solid became dissolved. The flask wasallowed to cool to room temperature, then placed in a 0° C. freezerovernight. The solid was collected by filtering through a filter-paperequipped Büchner funnel using vacuum filtration.

Preparation of BBIG-SO₄: A mixture of solid terephthalaldehyde (0.5mmol, 0.067 g), aqueous aminoguanidinium chloride (1.1 mmol, 2.2 mL, 0.5M), and water (10 mL) was stirred magnetically for 4 hours, whichresulted in a slightly yellow solution. Addition of sodium sulfate (0.5mmol, 0.5 mL, 1 M) to this solution resulted in instant precipitation ofa crystalline white solid. The crystalline solid was filtered after twoweeks and washed with water. Yield: 0.164 g (86%). HRMS (ESI-MS): m/zcalcd for C₁₀H₁₅N₈ ⁺: 247.14140; found: 247.14100; elemental analysiscalcd (%) for C₁₀H₂₀N₈O₆S: C, 31.58, H, 5.30, N, 29.46; found: C, 31.61,H, 5.53, N, 29.04. X-ray quality single crystals were obtained by slowevaporation of a solution containing aminoguanidinium chloride,terephthalaldehyde, and tetrabutylammonium sulfate in water/DMF. Thesimulated powder pattern from the single-crystal X-ray structuralanalysis matched the experimental PXRD pattern of bulk BBIG-SO₄precipitated from water.

Preparation of BBIG-NO₃: A mixture of solid terephthalaldehyde (0.5mmol, 0.067 g), aqueous aminoguanidinium chloride (1.5 mmol, 3 mL, 0.5M), and water (10 mL) was stirred magnetically for 5 hours, whichresulted in a slightly yellow solution. Addition of sodium nitrate (1mmol, 1 mL, 1 M) to this solution resulted in precipitation of acrystalline white solid after about 10 minutes. The mixture was stirredfor 12 hours, before the crystalline solid was filtered and washed withwater and ethanol. Yield 0.150 g (81%). HRMS (ESI-MS): m/z calcd forC₁₀H₁₅N₈ ⁺: 247.14140; found: 247.14130; elemental analysis calcd (%)for C₁₀H₁₆N₁₀O₆: C, 32.26, H, 4.33, N, 37.62; found: C, 32.57, H, 4.50,N, 6.64. X-ray quality single crystals were obtained by leaving themixture containing the initially precipitated solid undisturbed for twoweeks. The simulated powder pattern from the single-crystal X-raystructural analysis matched the experimental PXRD pattern of bulkBBIG-NO₃ precipitated from water.

Synthesis of 2,6-pyridine-bis(iminoguanidine) (PyBIG)

2,6-pyridine-bis(iminoguanidine) (PyBIG) was obtained by iminecondensation of 2,6-pyridinedialdehyde with aminoguanidinium chloride,followed by neutralization with aqueous NaOH, which led to precipitationof the pure compound as a crystalline hydrate (PyBIG.2.5H₂O). Thedetails of the synthesis are as follows:

Pyridine-2,6-dicarbaldehyde: A mixture of pyridine-2,6-dimethanol (10.00g, 71.86 mmol) and Dess-Martin periodinane (67.06 g, 158.10 mmol) weresuspended in 400 mL of dichloromethane. The reaction mixture was stirredat room temperature for 12 hours. Subsequently, 100 mL of water wasadded to the reaction and the mixture was stirred at room temperaturefor an additional 12 hours. A 300 mL 50/50 mixture of saturated NaHCO3and 10% Na₂S₂O₃ was added to the reaction mixture and stirred for twoadditional hours, then the mixture was filtered through a celite plug.The filtrate was poured into a separatory funnel and the organic layerwas collected, washed with brine, and dried with Na₂SO₄. Thedichloromethane was removed under vacuum resulting in a white-yellowsolid. The product was purified by column chromatography using thefollowing methodology. The solid was partly dissolved in hexanes andloaded onto a silica gel column (Note: the solid is not fully soluble inhexanes), and eluted using a hexanes-ethyl acetate solvent system (7:3hexanes/ethyl acetate). The final product was isolated as a white solid.Yield: 7.64 g, 78.7%. ₁H NMR (400 MHz, CDCl₃) δ 10.136 (2H, s), 8.164(2H, d), 8.070 (1H, t). ¹³C NMR (100 MHz, CDCl3) δ 192.38, 152.99,138.43, 125.36.

PyBIG.2HCl (2): Pyridine-2,6-dicarbaldehyde (7.64 g, 56.54 mmol) wassuspended in 200 mL of absolute ethanol, then aminoguanidinium chloride(13.75 g, 124.37 mmol) was added to the suspension. The reaction mixturewas mechanically stirred and heated to 65° C. for 8 hours (Note: as thereaction progresses, small amounts of the aldehyde dissolves and thebis-iminoguanidinium product precipitates out of solution as a whitesolid). After 8 hours, the product (PyBIG.2HCl) was obtained as a whitesolid by vacuum filtration of the reaction mixture, and washed withdiethyl ether to remove unreacted pyridine-2,6-dicarbaldehyde. Yield:90.8%. ¹H NMR (400 MHz, D₂O) δ 7.779 (1H, t), 7.717 (2H, s), 7.681 (2H,d). ¹³C NMR (100 MHz, D₂O) δ 154.96, 150.99, 145.20, 138.75, 123.51. Theforegoing product has the following structure:

PyBIG.2H₂O: PyBIG.2HCl (10.00 g, 31.23 mmol) was dissolved in 350 mL ofwater and the mixture was stirred for 1 hour to ensure completedissolution of the chloride salt. Subsequently, NaOH (6.9 mL, 10 M) wasadded, resulting in immediate precipitation of the free compound. Thesuspension was stirred for 4 hours, and then the pure PyBIG.2H2Ocompound was isolated as an off-white solid by vacuum filtration anddrying under vacuum at room temperature overnight. Yield: 96.1% (7.72g). ¹H NMR (400 MHz, DMSO-d₆) δ 7.999 (2H, s), 7.915 (2H, d), 7.647 (1H,t) 6.145 (4H, s) 5.924 (4H, s). ¹³C NMR (100 MHz, DMSO-d₆) δ 161.91,155.65, 143.88, 136.29, 118.19.

Solubility Measurements of BBIG-Cl, BBIG-SO₄, and BBIG-NO₃

The solubility of BBIG-Cl was determined gravimetrically. A saturatedsolution of BBIG-Cl was obtained by placing an excess of the salt in a15 mL polypropylene centrifuge tube and adding 2 mL of deionized water(milli-Q). The resulting suspension was mixed for 48 hours using arugged rotator set at 60 rpm, inside an incubator set at 258° C. After48 hours, the suspension was centrifuged for 10 minutes at 3000 rpm toseparate the aqueous and solid phases. The aqueous layer was thencarefully removed using a 0.22 μm syringe filter to remove any remainingsuspended solid from the solution. One mL of the saturated salt solutionwas then pipetted into a pre-weighed glass vial containing a magneticstir bar. The water was then removed under reduced pressure and gentleheating (˜50° C.) while stirring. The resulting solid was left undervacuum overnight to ensure complete removal of the water, prior toweighing the vial. The solubility measurements were run in triplicate,and the average weight of the recovered chloride salt was 0.0202 g,which corresponds to an aqueous solubility of 6.3(±0.2)×10⁻² M.

The solubilities of BBIG-SO4 and BBIG-NO3 were determined by UVspectroscopy. Prior to determining the solubility of these salts, acalibration curve was obtained using the more soluble BBIG-Cl salt.Saturated solutions of the BBIG-SO₄ and BBIG-NO₃ salts were prepared thesame way as for BBIG-Cl. These solutions were then diluted to ensure theconcentrations of the BBIG di-cation were in the concentration range ofthe calibration curve. The BBIG-NO₃ solutions were diluted 100-fold,whereas the BBIG-SO₄ solutions were diluted tenfold. The solubilitieswere then determined from the UV spectra of these diluted solutions bymeasurement of the absorbance maxima at 322 nm and comparison with thecalibration curve. The solubility measurements were run in triplicate,and the obtained averages and standard deviations for BBIG-SO₄ andBBIG-NO₃ were 1.6(2)×10⁻⁵ M and 6.5(5)×10⁻⁴ M, respectively.

Variable-Temperature Solubility Measurements of BBIG-SO₄

All measurements were done in triplicate and the reported solubilitiesare the average values. Excess amounts of BBIG-SO₄ were mixed with 10 mLof MilliQ water in 15 mL polypropylene centrifuge tubes. The resultingsuspensions were mixed for 72 hours using a rugged rotator set at 60rpm, inside an incubator set at 15, 20, 25, 30, or 35° C. Subsequently,the samples were removed and centrifuged for 10 minutes at 3000 rpm toseparate the aqueous and solid phases. A 3 mL aliquot was then removedfrom each sample for UV analysis. Then 3 mL of fresh MilliQ water wasadded to the samples to replace the aliquot of solution removed, and thesamples were mixed for an additional 72 hours at the next desiredtemperature before further sub-sampling. The temperatures weremaintained by using temperature controlled incubators containing NISTcertified thermometers. The 3 mL aliquots of subsampled solutions werefiltered through a 0.22 μm syringe filter to ensure any suspended solidwas removed from the solutions prior to diluting the samples using thesame dilution factors used in determining the solubilities at 258° C.,as described above. The solubilities were determined by UV spectrometry,as described in the previous section.

Competitive Crystallization of BBIG-SO₄ from an Aqueous Mixture ofSulfate, Nitrate, and Chloride

First, BBIG-Cl was generated in situ from terephtalaldehyde andaminoguanidinium chloride, as follows. Terephthalaldehyde (0.5 mmol,0.067 g), aminoguanidinium chloride (1.5 mmol, 3 mL, 0.5 M) and water(10 mL) were added to a 20 mL vial. The mixture was stirred at roomtemperature for 5 hours, which resulted in dissolution of most of thesuspended solid. A few drops of 1M HCl were then added to adjust the pHto around 5, which resulted in a clear, slightly yellow solution.Aqueous sodium sulfate (0.5 mmol, 0.5 mL, 1 M) and sodium nitrate (1mmol, 1 mL, 1 M) were then added, which resulted in the formation of awhite precipitate after about 2 minutes. The mixture was stirred at roomtemperature for 12 hours, before the crystalline solid was filtered andwashed with water. Yield: 0.190 g (100%). PXRD and FT-IR analysesconfirmed the crystallized solid was pure BBIG-SO₄.

Recovery of the BBIG Compound

BBIG-SO₄ (53.1 mg, 0.14 mmol) was added to a 2 mL solution of NaOH (10%)and the mixture was stirred for 2 hours at room temperature, whichresulted in the formation of a yellow precipitate. The solid wasfiltered using a pre-weighed filter paper, rinsed with 200 mL of water,then dried under vacuum. Yield: 31.8 mg (93%) as yellow powder. ¹H NMR(400 MHz, CD₃OD): δ=7.660 (s, 4H; CH), 8.015 ppm (s, 2H; N═CH).Dissolution of the yellow powder in 1 M HCl resulted in a clear solutionof BBIG-Cl, which could be reused for sulfate separation, asdemonstrated by precipitation of BBIGSO₄ upon addition of aqueous sodiumsulfate. X-ray quality single crystals of BBIG.2H₂O were obtained byslow evaporation of a solution containing a small amount of therecovered yellow powder dissolved into aqueous ethanol.

Sulfate Separation from Seawater

The seawater used in the experiment was collected from the gulf streamin the Atlantic Ocean. Prior to use, the water was pre-filtered toremove suspended particulates and small organisms. After filtration, 10mL of the ocean water was spiked with 96 mL of the ³⁵S radiotracer (asNa₂ ³⁵SO₄) for β liquid scintillation counting. The sulfateconcentration in seawater was estimated to be about 30 mM by titrationwith BaCl₂. Stock solutions of BBIG-Cl in MiliQ water were prepared,with concentrations of 15, 30, 33, 45, and 60 mM. A volume of 0.75 mL ofeach of these solutions was pipetted into a 2 mL Eppendorfmicrocentrifuge tube, and 0.75 mL of seawater pre-spiked with the ³⁵Sradiotracer was added. The resulting solution mixtures were mixed for 24hours using a rotating wheel set at 60 rpm in a temperature-controlledair-box set at 25±0.2° C. The tubes were then centrifuged for 10 minutesat 3000 rpm to separate the aqueous and solid phases, and 1 mL aliquotsolutions were removed using 0.22 μm syringe filters for β liquidscintillation counting.

Analysis of Sulfate Concentration by β Liquid Scintillation Counting

The radiolabeled ³⁵S radiotracer is a β emitter, thereby allowingdetermination of the sulfate concentration of a solution spiked with aknown amount of Na₂ ³⁵SO₄ by β liquid scintillation counting. Theseawater solutions were pre-spiked with 96 mL of the ³⁵S radiotracer, asabove. The amount of radiotracer used was based on the need to ensureapproximately 4.5 to 5 million initial counts per minute (CMP)/mL ofsolution (Ci mL⁻¹). The volume of the spike solution was determined byfactoring in the original activity of the solution and correcting forthe short half-life of the ³⁵S radiotracer. The Na₂ ³⁵SO₄ solution hadcompleted 3.8 half-lives before use in this experiment. The 1 mL aliquotsolutions removed from seawater (see above) were pipetted into 20 mL ofUltima Gold scintillation cocktail (PerkinElmer). It was necessary touse 20 mL of the cocktail to ensure complete solubility of the seawatersolutions in the cocktail. The resulting mixtures were vigorously shakento allow for complete dissolution and dispersion of the salt solutions.The samples were then placed on the analyzer and counted for 30 minutesafter allowing 60 minutes for dark-adaption.

Results and Discussion

FIG. 1A shows the single-crystal X-ray structural of BBIG-SO₄. As shownin FIG. 1A, BBIG-SO₄ has a virtually planar conformation for thebis(iminoguanidinium) cation, and includes two water molecules ofhydration in the crystal. As shown in the crystal structure perspectivein FIG. 1B, pairs of sulfate anions are linked together by four watermolecules into centrosymmetric [(SO₄)₂(H₂O)₄]⁴⁻ clusters. Each sulfateanion in the cluster accepts four water hydrogen bonds, with observed OH. . . 0 contact distances of 1.82, 1.84, 1.88, and 2.20 Å, and OH—Oangles of 169.8, 174.2, 156.6, and 167.38°, respectively. There are twocrystallographically distinct BBIG cations in the crystal; one isperfectly planar, whereas the other is slightly bent, with its terminalNH₂ groups deviating by 0.2 Å out of the mean plane of the cation. Asshown in the crystal structure perspective in FIG. 1C, the two cationsare stacked in an antiparallel fashion in an ABAB pattern in thecrystal, with a mean interplanar distance of 3.39 Å. The shortestintermolecular contacts between adjacent cations in the stacks are shownin inner and outer dashed lines in FIG. 1C, corresponding to contactsbetween the imine N atoms and the centroids of the benzene rings (3.35,3.48 Å), and between terminal NH₂ groups and the centers of the C═Nimine bonds (3.19, 3.33 Å), respectively. As shown by the more expansivecrystal structure in FIG. 1D, the anionic [(SO₄)₂(H₂O)₄]⁴⁻ clusters inthe crystal are flanked by four cationic BBIG stacks, accepting a totalof 20 NH . . . O hydrogen bonds from the guanidinium groups, of which 14are to the sulfate anions, and 6 to the water molecules in the cluster.Thus, the total coordination number of each sulfate anion is 11,consisting of 7 NH . . . O hydrogen bonds from guanidinium groups, and 4OH . . . O hydrogen bonds from water.

The X-ray crystal structure of BBIG-NO₃ is shown in FIG. 2A, with thestacking arrangement perspective shown in FIG. 2B. As in the analogoussulfate structure, the BBIG cations are stacked within the crystal,although in this case they are oriented parallel to each other, with amean interplanar distance of 3.27 Å between adjacent cations in thestack. As shown in the more expansive view of FIG. 2C, the nitrateanions link the stacks into a three-dimensional hydrogen-bonded network,with each anion accepting five hydrogen bonds from three neighboringguanidinium groups.

Effective aqueous anion separation by crystallization of guanidinesrequires in the first place that the guanidinium salt of the targetedanion is relatively insoluble in water. For the crystallization to beselective, the guanidinium salt of the targeted anion also needs to besignificantly less soluble than the corresponding salts of the competinganions. Table 1, below, lists the measured aqueous solubilities of thesulfate, nitrate, and chloride salts of BBIG.

TABLE 1 Aqueous solubilities of different BBIG salts at 25° C. BBIG SaltSolubility (M) sulfate^(a) 1.6(2) × 10⁻⁵ nitrate^(a) 6.5(5) × 10⁻⁴chloride^(b) 6.3(2) × 10⁻² ^(a)Measured by UV spectroscopy, ^(b)Measuredgravimetrically

The aqueous solubility of the sulfate salt was found to be lower thanthe corresponding solubilities of the nitrate and chloride analogues, bya factor of about 40 and 4000, respectively. Notably, the solubility ofBBIG-SO₄ is also lower by a factor of 45 than the solubility of theglyoxal-bis-(iminoguanidinium) sulfate salt, as previously reported (R.Custelcean, et al., Angew. Chem. Int. Ed. 2015, 54, 10525; Angew. Chem.2015, 127, 10671). The corresponding solubility product (K_(sp)) ofBBIG-SO₄ is 2.4(±0.6)×10⁻¹⁰, which is only marginally higher than theK_(sp) of BaSO₄ (1.1×10⁻¹⁰). Variable-temperature dissolutionmeasurements indicated that the solubility of BBIG-SO₄ slightlydecreases with increasing temperatures. From the van't Hoff plot shownin FIG. 3, it can be seen that the enthalpy of dissolution, as obtainedfrom the slope of the plot, is −3.7(±0.8)×10⁻¹⁰ kJmol⁻¹. Thus,crystallization of BBIG-SO₄ is slightly endothermic and entropy driven.The exceptionally low aqueous solubility of BBIG-SO₄ is quite unusualfor a guanidinium sulfate salt. This low solubility implies highstability for the BBIG-SO₄ crystals.

Electronic-structure calculations using density functional theory (DFT)indicated the stacking interactions between the bis-iminoguanidiniumcations in the BBIG-SO₄ crystals are mainly electrostatic in nature. Theelectrostatic potential maps of the BBIG cation, either in the BBIG-SO₄crystal, or isolated in the gas phase, showed that the C atoms,including those of the phenyl ring, tend to be electropositive, whereasthe N atoms of the guanidinium and imine groups are all electronegative.The atomic charges of the BBIG cation were calculated using the Baderscheme. These charges are generally consistent with the relative offsetof the BBIG cations observed in the BBIG-SO4 crystals (FIG. 1C), so thatthe closest intercationic contacts are between the terminal N atoms ofthe guanidinium groups (−1.31 charge) and the C atoms of the iminegroups (+0.77 charge), and between the imine N atoms (−0.75 charge) andthe C atoms of the Ph ring (+0.21, +0.13 charges). It thus appears thatthe stacking of the BBIG cations in these crystals is determined to alarge extent by complementary electrostatic attractions between positiveand negative regions of the planar cations.

Consistent with the measured aqueous solubilities that showed thesulfate salt was the least soluble in the series, crystallization ofBBIG-SO₄ from an aqueous mixture containing chloride (0.1M), nitrate(0.07M), and sulfate (0.034M) proved highly selective, resulting inexclusive separation of the sulfate anion in quantitative yield. TheBBIG compound was easily recovered by deprotonation of the guanidiniumgroups with 10% aqueous NaOH, which resulted in crystallization of theneutral BBIG compound in 93% yield. The compound can be recycled byconverting it back into the cationic form with aqueous HCl. The overallsulfate separation cycle is provided in FIG. 4.

To demonstrate the real-world utility of this sulfate separation method,the removal of sulfate from seawater by selective crystallization ofBBIG-SO₄ was attempted. The presence of relatively high concentrationsof sulfate in seawater (˜30 mM) poses significant scale problems in oilfield injection operations. Once formed, the sulfate scale deposits (asCaSO₄, SrSO₄, and BaSO₄) are difficult to remove and cause majoroperational problems with high remedial costs, and in some cases, resultin irreversible damage and well shutdown. It is, therefore, highlydesirable to prevent the scale problems by removing sulfate fromseawater.

Table 2, below, shows the results from the sulfate separation fromseawater by crystallization of BBIG-SO₄. The sulfate concentration insolution was monitored by using radiolabeled Na₂ ³⁵SO₄ and β liquidscintillation counting, an analytical method typically used inliquid-liquid extractions, and recently demonstrated to also beeffective in crystallization-based sulfate separations (R. Custelcean,et al., Cryst. Growth Des. 2015, 15, 517). Crystallization of BBIG-SO₄from seawater proved very efficient, with 99% of sulfate being removedby using only 1.5 molar equivalents of the BBIG cation.

TABLE 2 Sulfate separation from seawater^(a) Amount of BBIG [SO₄ ²⁻]left SO₄ ²⁻ removed [equiv]^(b) [mM]^(c) [%] 1 3.5 88 1.1 1.6 95 1.5 0.399 2 0.3 99 ^(a)Seawater from the Gulf Stream; the initial sulfateconcentration was estimated at 30 mm by titration with BaCl₂; ^(b)Molarequivalents of the BBIG dichloride salt added relative to the sulfate inseawater. ^(c)Corresponding sulfate concentration left in the seawater,measured by using radiolabeled Na₂ ³⁵SO₄ and β liquid scintillationcounting

The above experimental results demonstrate an effective approach toaqueous sulfate separation by selective crystallization using animine-linked bis-guanidinium compound self-assembled in situ from simplebuilding blocks. The high sulfate crystallization efficiency stems fromthe exceptionally low aqueous solubility of the BBIG-SO₄ salt, which issignificantly lower than the aqueous solubility of most, if not all,known organic sulfate salts, and comparable to that of BaSO₄.Furthermore, compared to precipitation with BaCl₂, thecrystallization-based approach described here offers a greeneralternative to aqueous sulfate separation that circumvents the use oftoxic barium.

An important factor in the stability of the BBIG-SO₄ crystals appears tobe the favorable stacking of the rigid and planar bis-iminoguanidiniumcations, which are arranged to optimize the electrostatic attractionbetween the positive and negative areas of the cationic compounds.Another structural factor likely to play a key role in the lowsolubility of the BBIG-SO₄ crystals and the high sulfate crystallizationselectivity is the sequestration of the sulfate anions as[(SO₄)₂(H₂O)₄]⁴⁻ clusters and their complementary hydrogen bonding bythe guanidinium groups. However, ultimately, the BBIG-SO₄crystallization is entropy driven, presumably reflecting theentropically favorable release of water molecules from the stronglyhydrated sulfate anions and the planar BBIG cations. Thus, this exampleof selective sulfate crystallization as sulfate-water clustersrepresents a complex recognition phenomenon that extends far beyond thesimple lock-and-key principle commonly invoked in supramolecularchemistry. The selective crystallization involves a multitude offactors, including the mutual recognition of molecular and ioniccomponents, a fine interplay of enthalpy and entropy, and a series ofbinding, self-assembly, and solvent exchange events that lead in the endto the nucleation and growth of highly insoluble crystals.

Compounds (1) and (2), in particular, exhibit very high degrees ofseparation of sulfate from seawater. The results are shown in Table 3,below.

TABLE 3 Sulfate removal abilities of Compounds 1 and 2 Initial Sulfate(mmol/L) 1 (mmol/L) % Sulfate Removed Sulfate Left (mg/L) 30 15 40.81706 30 15 41.8 1676 30 30 88.0 347 30 30 88.7 326 30 33 95.6 128 30 3393.6 185 30 45 99.0 28 30 45 98.9 30 30 60 98.9 31 30 60 98.9 31 Sulfate(mmol/L) 2 (mmol/L) % Sulfate Removed Sulfate Left (mg/L) 30 15 48.411487 30 15 48.57 1482 30 30 93.28 194 30 30 94.41 161 30 33 99.72 8 3033 99.60 12 30 45 99.94 1.7 30 45 99.94 1.7 30 60 99.95 1.5 30 60 99.951.5

The invention can be broadly applied to sulfate removal from aqueoussolutions of various compositions, as well as from insoluble solids(e.g., sulfate scale) or solid suspensions (e.g., sulfate-containingsolid suspensions in blackstrap molasses). Dissolution of insolublesulfate salts, such as calcium sulfate (CaSO₄), can be achieved in atwo-step process consisting of sequential dissolution of calcium andsulfate ions.

An example of sulfate removal from a calcium sulfate suspension is shownschematically in FIG. 5. In the first step, the initial CaSO₄ solid istreated with an aqueous chloride salt of the compound that results incrystallization of the guanidinium sulfate salt and release of thecalcium ions in solution, as soluble CaCl₂. In the second step, theinsoluble guanidinium sulfate salt is treated with sodium hydroxide baseto neutralize the guanidinium cations and release the sulfate ions insolution, as soluble Na₂SO₄. The resulting solid guanidine compound isthen recycled and converted back into guanidinium chloride with aqueousHCl, so it can be reused in another CaSO₄ dissolution cycle. Thus, theentire process is done in water (no organic solvents used), and the onlychemicals consumed in the overall process are NaOH and HCl, resulting intwo aqueous waste streams consisting of calcium chloride and sodiumsulfate. The unique aspect of this invention is the employment of acrystallization process to achieve dissolution of CaSO₄. Also, thisapproach is based on anion chelation by guanidinium groups rather thanthe more common metal chelation, which may be prone to interference fromother metals in solution. The particular process shown in FIG. 5 employsthe 1,4-benzene-bis(iminoguanidine) compound (BBIG) that has herein beenfound to form a sulfate salt (BBIG-SO4) of extremely low aqueoussolubility. Single crystal X-ray diffraction indicated that this salt ishydrated by two water molecules in the crystalline state (FIG. 1A). Thewater-soluble chloride salt of this guanidine compound (BBIG-Cl) reactswith a suspension of CaSO₄ in water and forms crystalline BBIG-SO₄,while the calcium cations are released into the aqueous solution asCaCl₂. In the preliminary tests, the reaction was run overnight at roomtemperature and pH 6-7, and was monitored by powder X-ray diffraction,which confirmed the complete conversion of solid CaSO₄(H₂O)₂ intocrystalline BBIG-SO₄(H₂O)₂. Subsequently, the crystals of BBIG-SO₄ weresuspended in 10% aqueous sodium hydroxide and stirred at roomtemperature for two hours to yield solid BBIG (confirmed by X-raydiffraction as BBIG(H₂O)₂) and aqueous sodium sulfate. Finally, therecovered BBIG was converted back into the chloride salt by treatmentwith aqueous hydrochloric acid and recycled for another CaSO₄dissolution cycle.

Direct Air Capture of Carbon Dioxide

Negative emissions, i.e. the net removal of greenhouse gases from theatmosphere, are now considered essential for stabilizing the globaltemperature at an optimal level. Direct air capture (DAC) of carbondioxide from ambient air is one of the few available options forlowering the atmospheric CO₂ concentration, but existing technologiestend to be energy demanding and prohibitively expensive. Herein wereport an effective and sustainable approach to DAC using theabove-described bis-iminoguanidine and bis-iminoguanidinium compoundsaccording to Formulas (1) and (1a), and more specifically, compounds 1and 2.

Direct air capture of CO₂ with glycine and sarcosine: The CO₂ absorptionfrom air was carried out with an Envion Humidiheat™ household airhumidifier. The humidifier consists of a reservoir with a capacity of ˜2L, a rotating wick, which is made of a porous fabric that absorbs theliquid from the reservoir and provides a larger surface area, and a fan.The reservoir was filled with 1.5 L aqueous solutions of glycine orsarcosine (1 M) and KOH (1 M) and the fan was run on slow setting,corresponding to an air flow rate of 3.8±0.2 m/s. The captureexperiments were run at ambient temperature (21±1° C.). However becauseof the water evaporation, the solution temperature was lower, averaging16±1° C. The reservoir was replenished periodically with H₂O tocompensate for the evaporated water (on average the evaporation rate was100 mL/h) and keep the amino acid concentration as constant as possible.The change in the pH of the amino acid solution was monitored in situwith a glass electrode. The amount of CO₂ absorbed was monitored bywithdrawing 300 μL samples and analyzing their carbonate and carbamatecontent by ion chromatography (IC) and ¹H NMR spectroscopy,respectively. For NMR analyses, 900 μL of D₂O was added to 100 μL of thesamples, whereas for the IC analyses, the samples were diluted 10-300fold to bring the carbonate concentration in the 30-300 ppm range.

Regeneration of the amino acid sorbents with PyBIG: All regenerationswere carried out at 25° C. in a thermostated oven. The amino acidsolutions (5 mL) were placed in 15 mL polypropylene centrifuge tubes andPyBIG.2.5H₂O was added as a solid. The amount of PyBIG added varied withthe CO₂ loading of the solution; the optimum amount was found to be 0.5molar equivalents relative to the CO₂ absorbed (moles CO2/molesPyBIG=2). The resulting suspensions were mixed on a rotating wheel at 60rpm to allow for a good contact between the two phases. Sub-samples werewithdrawn hourly for the first 4 hours and then overnight. Each time,the tubes were centrifuged at 4000 rpm for 3 to 4 minutes depending onthe thickness of the slurry. 50 μL of solution was then withdrawn usinga micropipette to prepare the NMR and IC samples. For NMR analyses, 10μL of the recovered solutions were mixed with 500 μL of D₂O, and for ICanalyses, 5-10 μL of the solutions were diluted with 900 to 950 μL ofH₂O. At the end of the regenerations, the final solids were filtered andanalyzed by PXRD for phase identification.

Carbon dioxide release and regeneration of PyBIG using concentratedsolar power: The CO₂ release from the PyBIGH₂(CO₃)(H₂O)₄ crystals wascarried out by solar heating with a solar oven. The oven consists of avacuum-insulated borosilicate tube placed in the focal point of twoadjustable parabolic reflectors. The temperature inside the tube wasmonitored with a thermocouple. The PyBIGH₂(CO₃)(H₂O)₄ samples (0.038 g,0.1 mmol) were loaded in 1 mL glass vials, which were placed inside theoven tube. The solar oven was then placed in the full sun and orientedto capture the maximum amount of sunlight. The temperature was ramped tothe targeted values of 120° C., 140° C., 150° C., or 160° C. as fast aspossible (typically within 3 to 10 minutes), then held within ±2° C. byintermittently moving the oven out of the sun, or/and closing thereflectors. The samples were subsequently removed from the tube, allowedto cool to room temperature, and weighed to determine their mass loss.The resulting yellow solids were analyzed by FTIR to confirm thedisappearance of the carbonate and water peaks.

Preliminary results indicated that aqueous2,6-pyridine-bis(iminoguanidine) (PyBIG) captures CO₂ from ambient airand binds it as a crystalline tetrahydrated carbonate saltPyBIGH₂(CO₃)(H₂O)₄. The CO₂ can be released by heating the carbonatecrystals at relatively mild temperatures of 80-120° C., whichregenerates the PyBIG compound quantitatively. A general schematic ofthe process is shown in FIG. 6. Examination of the PyBIGH₂(CO₃)(H₂O)₄crystals by optical microscopy revealed that, upon heating in an oven at120° C. for one hour, the crystals changed their color from cream toyellow to opaque. Thermogravimetric analysis coupled with massspectrometry (TGA-MS) provided a more quantitative picture of thedecomposition process. In a temperature-ramped TGA measurement, thePyBIGH₂(CO₃)(H₂O)₄ crystals lost 35.2% of their mass between 65 and 140°C., and the MS analysis confirmed the simultaneous evolution of waterand CO₂. These measurements are consistent with the loss of onecarbonate and two protons (as CO₂ and H₂O), and four additional watermolecules, as expected from the crystal structure of PyBIGH₂(CO₃)(H₂O)₄(35.1% theoretical mass loss). Similarly, the mass loss of the crystalsheated in the oven for one hour at 120° C. (vide supra) was 34.3%, andthe FTIR and NMR spectroscopic analysis of the resulting solid confirmedthe complete disappearance of the carbonate peak and the regeneration ofthe anhydrous PyBIG compound. The TGA-MS analysis showed nodecomposition of the regenerated compound up to 190° C., which providesa thermal stability window of at least 50° C. for compound recovery.Isothermal TGA runs at 120 and 100° C. showed complete loss of carbondioxide and water after 60 and 150 minutes, respectively, with noadditional mass loss after 5 hours. On the other hand, at 80° C. thedecomposition reached 77% completion after 300 minutes. This correspondsto about an order of magnitude reduction in the decompositiontemperature compared to inorganic carbonates, such as Na₂CO₃ or CaCO₃,involved in traditional DAC technologies.

The elementary steps involved in the CO₂ absorption and the overallreaction are represented by equations 1-7 as follows:

$\begin{matrix}\left. {PyBIG}_{(s)}\rightleftharpoons{PyBIG}_{({aq})} \right. & (1) \\\left. {{PyBIG}_{({aq})} + {2\mspace{14mu} H_{2}O}}\rightleftharpoons{{PyBIGH}_{2{({aq})}}^{2 +} + {2{HO}_{({aq})}^{-}}} \right. & (2) \\\left. {CO}_{2{(g)}}\rightleftharpoons{CO}_{2{({aq})}} \right. & (3) \\\left. {{CO}_{2{({aq})}} + {HO}_{({aq})}^{-}}\rightarrow{HCO}_{3{({aq})}}^{-} \right. & (4) \\\left. {{HCO}_{3{({aq})}}^{-} + {HO}_{({aq})}^{-}}\rightleftharpoons{{CO}_{3{({aq})}}^{2 -} + {H_{2}O}} \right. & (5) \\\left. {{PyBIGH}_{2{({aq})}}^{2 +} + {CO}_{3{({aq})}}^{2 -} + {4\mspace{14mu} H_{2}O}}\rightleftharpoons{{{PyBIGH}_{2}\left( {CO}_{3} \right)}\left( {H_{2}O} \right)_{4{(s)}}} \right. & (6) \\\left. {{PyBIG}_{(s)} + {CO}_{2{(g)}} + {5\mspace{14mu} H_{2}O}}\rightarrow{{{PyBIGH}_{2}\left( {CO}_{3} \right)}\left( {H_{2}O} \right)_{4{(s)}}} \right. & (7)\end{matrix}$

The crystalline PyBIG compound (as PyBIG.2H2O hydrate) first dissolvesinto water (equation 1), then the two guanidine groups become protonatedby water molecules, generating the dicationic form of the compound(PyBIGH₂ ²⁺) and HO⁻ (equation 2). The hydroxide anions are the actualactive species that react with the CO₂ absorbed from air (equation 3)and generate bicarbonate (equation 4) and carbonate anions (equation 5).Finally, the PyBIGH₂ ²⁺ and CO₃ ²⁻ ions crystallize with water intocrystalline PyBIGH₂(CO₃)(H₂O)₄ (equation 6). The net reaction, shown inequation 7, corresponds to crystalline PyBIG converting into crystallinePyBIGH₂(CO₃)(H₂O)₄, in the presence of CO₂ and water, throughdissolution/recrystallization.

Although PyBIG can capture CO₂ from ambient air according to equation 7,the reaction is too slow for practical considerations. The kinetics ofCO₂ absorption by aqueous alkaline solutions, such as NaOH, have beenfound to be limited by a combination of the CO₂ diffusion into theaqueous solution (equation 3), and the reaction of CO₂ with HO—(equation 4). Thus, the rate of CO₂ absorption is controlled by thesurface area of the air-liquid interface, and the solution alkalinity.For a typical 1 M solution of NaOH, the flux of CO₂ absorbed from airhas been estimated around 30 μmol/m2/s (Zeman, F., Environ. Sci.Technol. 41, 7558-7563, 2007). However, compared to NaOH, PyBIG issignificantly less alkaline. A saturated solution of PyBIG (˜10 mM) hasa pH of about 10, which corresponds to a rate of CO₂ reaction that is atleast a couple orders of magnitude lower than for NaOH. Anotherconstraint is that in a typical crystallization set-up the air-liquidcontact area is relatively small, which further limits the CO₂absorption rate.

One possible solution to the slow CO₂ sorption problem is to combine thePyBIG crystallization with a traditional aqueous sorbent that absorbsatmospheric CO₂ relatively fast and converts it into carbonate. Thesolution is subsequently reacted with PyBIG to crystallizePyBIGH₂(CO₃)(H₂O)₄ and regenerate the sorbent. Finally, the carbonatecrystals are filtered out of solution and heated in the solid state torelease the CO₂ and regenerate the PyBIG compound, which can then bereused in another cycle. FIG. 7A provides a general schematic of theabove concept. The advantage of such a hybrid approach to CO₂ capture,which combines room temperature absorption in the liquid phase with CO₂release in the solid state, is that it benefits from the fast sorptionkinetics of an aqueous sorbent while avoiding the energy penaltyassociated with heating aqueous solutions during regeneration.Furthermore, sorbent loss through evaporation and thermal degradation isminimized.

One approach to DAC with PyBIG is to combine the crystallization ofPyBIGH₂(CO₃)(H₂O)₄ with the well-established carbonate/bicarbonate CO₂capture cycle. A general schematic of the process is shown in FIG. 7B.The chemical reactions occurring in the carbonate/bicarbonate CO₂capture cycle are shown in the following equations:

CO₃ ²⁻+CO₂+H₂O→2 HCO₃ ⁻  (Eq. 1)

PyBIG_((s))+2HCO₃ ⁻+4H₂O→PyBIGH₂(CO₃)(H₂O)_(4(s))+CO₃ ²⁻  (Eq. 2)

In this approach, CO₂ sorption by an alkali carbonate solution (Eq.1) isfollowed by the reaction of the resulting bicarbonate with PyBIG tocrystallize PyBIGH₂(CO₃)(H₂O)₄ and regenerate the carbonate sorbent(Eq.2). Finally, thermal decomposition of PyBIGH₂(CO₃)(H₂O)₄ regeneratesthe PyBIG compound and releases the CO₂. To demonstrate the feasibilityof this approach, solid PyBIG (1 mol equiv) was suspended in a solutionof 1 M NaHCO₃ (5-6 mol equiv) and the slurry was stirred at roomtemperature for four hours. The resulting mixture was filtered, and theseparated crystalline solid was confirmed by PXRD and FTIR to bePyBIGH₂(CO₃)(H₂O)₄. Subsequent heating of the carbonate crystals in theoven for one hour at 120° C. regenerated the PyBIG solid, which wasrecycled back into the original sodium bicarbonate solution. The entirecarbonate separation cycle was run three times, with observed yields forPyBIGH₂(CO₃)(H₂O)₄ crystallization of 99.0±0.4%, 97.2±0.6%, and91.4±0.4%, corresponding to the first, second, and third cycle,respectively. The regeneration of the PyBIG compound was nearlyquantitative in each cycle. The slight decrease in the crystallizationyield observed in the later cycles is explained by the gradual increasein the solution alkalinity (initial pH 8.5, final pH 10.5) as a resultof the increasing CO₃ ²⁻/HCO₃ ⁻ ratio. As more bicarbonate is convertedinto carbonate in each subsequent cycle, according to Eq. 2, the pH ofthe solution should eventually become high enough to inhibit theprotonation of PyBIG, thereby decreasing the driving force for thecrystallization of PyBIGH₂(CO₃)(H₂O)₄. This is corroborated by the FTIRanalysis of the isolated solid, which showed preponderantly thecarbonate phase after the first two cycles, but a mixture of carbonateand free PyBIG compound after the third cycle.

After considering several potential sorbents, two simple amino acids,glycine and sarcosine, were selected for the DAC system. Aqueous aminoacids have a number of positive attributes that make them promisingcandidates for DAC. They have fast CO₂ sorption kinetics, on par with orsurpassing more traditional sorbents like monoethanolamine or NaOH.Amino acids are non-volatile, non-corrosive, environmentally friendly,and relatively inexpensive. They are also less susceptible to oxidationthan amines. While amino acid sorbents have been employed in CO₂scrubbing from flue gas, their use in DAC remains unexplored. Thechemical reactions involved in the CO₂ absorption with amino acids andin the sorbent regeneration with PyBIG are depicted in equations 8-10below, using glycine as a representative example.

First, the anionic form of glycine (glycinate) reacts with CO₂ andgenerates the corresponding carbamic acid, which is deprotonated by asecond equivalent of glycine to generate the carbamate and thezwiterionic glycine (equation 8). The carbamate is subsequentlyhydrolyzed to glycinate and bicarbonate (equation 9). Finally, PyBIGdeprotonates the bicarbonate and the zwiterionic glycine andcrystallizes as PyBIGH₂(CO₃)(H₂O)₄ while regenerating the glycinatesorbent (equation 10). Note that adding the three reactions togetherleads to the same overall reaction represented by equation 7. However,the kinetics of the amino acid-mediated DAC process are expected to besignificantly faster than with PyBIG alone.

In addition to fast kinetics for the CO₂ reaction with the sorbent, apractical DAC system requires effective mass transfer of CO₂ from airinto the sorbent solution, which in turn requires an efficient contactorthat maximizes the air-liquid interfacial area. Unlike CO₂ capture fromflue gas, which is typically done with a packed absorption column thatis designed to operate at high liquid/gas ratios and with a high degreeof CO₂ removal, DAC is more suitably done in a more open system withcontactors that are optimized to ingest large volumes of ambient air, inmany ways similar in design with large-scale cooling towers. For thepurpose of this study, which is a small-scale proof of principle for DACwith the amino acid/guanidine system, a household humidifier was used asan air-liquid contactor. Like a cooling tower, an air humidifier isdesigned to maximize the air-water contact area, and by replacing thewater with an amino acid solution, it effectively becomes a DACabsorber.

Thermodynamic Analysis

Once the basic design elements were established, the next important stepwas to determine the thermodynamics of the CO₂ absorption anddesorption, as well as of sorbent regeneration, which define the energyand efficiency boundaries for the DAC system. The energetics of the CO₂absorption (equation 7) can be obtained by adding up the energetics ofthe elementary reactions represented by equations 1-6. The enthalpies ofCO₂ hydration (equation 3) and of reaction with HO— to generatebicarbonate (equation 4) are already known. The heat of bicarbonatedeprotonation by HO— to make CO₃ ²⁻ (equation 5) can also be calculatedfrom published thermodynamic data on carbonate protonation and waterionization. The remaining enthalpies for the reactions involving PyBIGand PyBIGH₂(CO₃)(H₂O)₄ were determined as part of this study. Theenthalpies of PyBIG dissolution (equation 1) and PyBIGH₂(CO₃)(H₂O)₄crystallization (equation 6) were obtained by variable-temperaturesolubility measurements of the two solids and van't Hoff analyses).Finally, the enthalpies of PyBIG protonation (equation 2) were obtainedfrom variable-temperature pKa measurements by potentiometric titrationsand van't Hoff analyses. The corresponding ΔH values for reactions 1-6are listed in Table 4 below. Adding up these values results in anoverall enthalpy of CO₂ absorption by PyBIG of −70.7 kJ/mol.

TABLE 4 Reaction enthalpies for the corresponding elementary stepsinvolved in DAC with PyBIG. Reaction ΔH (kJ/mol) Reference 1 42.5 Thisstudy 2 43.6^(a) This study 3 −19.4 29 4 −50 30 5 −40.4 31, 32 6 −47This study ^(a)The enthalpy for reaction 2 is composed of the sum of thefirst (−31 kJ/mol) and second (−37 kJ/mol) protonation enthalpies ofPyBIG minus twice the enthalpy of ionization of water (55.8 kJ/mol).

Although the CO₂ desorption could theoretically be achieved by runningreactions 1-6 in reverse order according to the principle of microscopicreversibility, in reality it would not be feasible as reaction 4 ispractically irreversible. Such an approach would also defeat the purposeof avoiding heating the aqueous sorbent. Instead, once the loading iscomplete, the crystalline PyBIGH₂(CO₃)(H₂O)₄ is removed from solutionand heated in the solid state to release the CO₂ gas and the watervapors. Given the completely different conditions involved, such agas-solid process must have different energetics compared to thegas-liquid-solid process involved in CO₂ absorption. In order todetermine the enthalpy of CO₂ release, differential scanning calorimetry(DSC) was employed. DSC is a common technique used in thermal analysisof solids. The DSC curve (showed a series of endothermic events between80 and 140° C., corresponding to the release of water and CO₂ aspreviously found by thermogravimetric analysis (Seipp et al., Angew.Chem. Int. Ed. 56, 1042-1045, 2017). Unfortunately, the extensiveoverlap between the peaks prevented a determination of the heatsassociated with each thermal event. Instead, all the peaks wereintegrated together to obtain the overall enthalpy of desorption forPyBIGH₂(CO₃)(H₂O)₄, which amounts to 223±4 kJ/mol. While the overallreaction is highly endothermic, it must be taken into account that foreach mole of CO₂ released, there are five moles of water that need to bevaporized. As the enthalpy of vaporization for water is 40.65 kJ/mol,more than 90% of the enthalpy of desorption is used for waterevaporation. This represents a significant energy penalty that must beaccounted for, and which encourages the use of renewable sources ofenergy, such as concentrated solar power (vide infra), to make theoverall DAC process energy more sustainable.

In addition to the enthalpies of CO2 absorption and release, anotherimportant thermodynamic parameter is the equilibrium constant for theamino acid regeneration reaction (equation 10), which defines theefficiency limit for sorbent regeneration. In the regeneration reaction,the solid PyBIG has to dissolve into the sorbent solution, deprotonatethe amino acid and the bicarbonate ion, and crystallize asPyBIGH₂(CO₃)(H₂O)₄. In the case of the glycine sorbent, the equilibriumconstant for the regeneration reaction (log Kreg) is defined by equation11 as follows:

log K_(reg)=log K_(sp)(PyBIG)−logK_(sp)(PyBIGH₂)(CO₃)(H₂O)₄)−pK_(a)(Gly)−pK_(a)(HCO₃⁻)+pK_(a1)(PyBIG)+pK_(a2)(PyBIG)  (11)

Thus, the amino acid regeneration is driven by the difference insolubility between PyBIG and PyBIGH₂(CO₃)(H₂O)₄, as well as thedifference in basicity between glycine and bicarbonate on one hand, andPyBIG on the other hand. The pKa values at 25° C. for the twoguanidinium groups of PyBIG, determined by potentiometric titration, are7.6 and 8.7. On the other hand, the pKa values of glycine andbicarbonate are 9.5 and 10.3, respectively (Yang N. et al., Ind. Eng.Chem. Res. 53, 12848-12855, 2014). Thus, PyBIG is not sufficiently basicto drive the regeneration equilibrium to the right, and therefore themain driving force has to come from the solubility difference betweenPyBIG and PyBIGH₂(CO₃)(H₂O)₄. A preliminary estimated value of 1.0×10⁻⁸is given for the solubility product of PyBIGH₂(CO₃)(H₂O)₄. As part ofthis study, determination of a more accurate value became possible asthe exact speciation of PyBIG in solution could now be obtained based onthe measured pKa values of PyBIG, resulting in a revised K_(sp) forPyBIGH₂(CO₃)(H₂O)₄ of 1.0±0.3×10⁻⁹ at 25° C. This corresponds to a veryinsoluble carbonate salt, on a par with the natural calcite mineral(K_(sp)=3.3×10⁻⁹). By comparison, the measured K_(sp) of PyBIG at thesame temperature is 1.0±0.3×10⁻². Thus, under ideal conditions, logK_(reg)=3.5, which predicts a very efficient amino acid regeneration.However, under realistic conditions involving high ionic strengthsolutions that can significantly impact the solubilities of the variousspecies involved through ion pairing, salting out, etc., the observedregeneration efficiency may actually be less than ideal.

Carbon Dioxide Absorption with Aqueous Glycine and Sarcosine

The direct air capture of CO₂ in this study was done with an airhumidifier shown using aqueous amino acid sorbents (as potassium salts).The loading of CO₂ into 1 M aqueous solutions of potassium glycinate andsarcosinate as a function of time is shown in FIG. 8. The extent of CO₂absorption was monitored in situ by pH measurements, and ex situ by ICand ¹H NMR to determine the amounts of carbonate and carbamate formed.The sorption experiments were run for 24 hours and the results aresummarized in Table 5 below.

TABLE 5 Carbon dioxide loading values for 1M aqueous solutions ofpotassium glycinate and sarcosinate after 24 hours pH CarbonateCarbamate Sorbent (initial/final) (M) (M) Total CO₂ (M) Glycine (1M)12.31/9.52 0.46 0.27 0.73 Sarcosine (1M) 12.92/9.99 0.46 0.17 0.63

The initial CO₂ absorption rate is slightly higher for sarcosine thanglycine, possibly reflecting the more alkaline nature of the former.However, the absorption rate for sarcosine slows down considerably andlevels off after about 6 hours, while the corresponding absorption ratefor glycine slows down more gradually. While the reaction rates of CO₂with both sarcosine and glycine are expected to be quite high (18.6 and13.9 kM/s, respectively, at 25° C.), the much slower rates of CO₂absorption observed presumably reflect the relatively limited (<1 m²)air-water interfacial area available with the air humidifier employed inthis study. After 24 hours, the total CO₂ loading for glycine (0.73mol/mol) was slightly higher than for sarcosine (0.63 mol/mol). Thedifference can be accounted for by the larger amount of carbamate formedwith glycine (0.27 M) compared to sarcosine (0.17 M).

Sorbent Regeneration by Carbonate Crystallization with PyBIG

The CO₂-loaded sorbents were stirred with a suspension of PyBIG (0.5molar equivalents relative to CO₂) at room temperature, which resultedin crystallization of PyBIGH₂(CO₃)(H₂O)₄ and regeneration of the anionicforms of the amino acids, according to equation 10. The formation ofcrystalline PyBIGH₂(CO₃)(H₂O)₄ was confirmed by powder X-ray diffraction(PXRD), which revealed after 24 hours a mixture of PyBIG.2.5H₂O and thecarbonate salt. The concentrations of carbonate and carbamate in theamino acid solutions were monitored by IC and NMR, respectively, and thedecrease in total CO₂ loading as a function of time is plotted in FIG.9. The results after 24 hours regeneration time are summarized in Table6 below.

TABLE 6 Regeneration results for the 1M aqueous solutions of potassiumglycinate and sarcosinate after stirring with a suspension of PyBIG (0.5molar equiv) for 24 hours. Carbonate Carbamate Total Sorbent pH(initial/final) (M) (M) CO₂ (M) Glycine (1M) 9.63/10.33 0.33 0.05 0.38Sarcosine (1M) 9.99/10.53 0.29 0.03 0.32

Most of the CO₂ from each sorbent is released within an hour, and longerregeneration times only led to marginal improvements. The cycliccapacity, defined as the difference between the maximum CO₂ loadingobserved after absorption and the minimum CO₂ loading measured afterregeneration, is 0.35 and 0.31 mol/mol for the glycine and sarcosinesorbents, respectively.

Regeneration of the amino acid sorbents can also be achieved in atraditional fashion, by boiling the aqueous solutions under reflux.After one hour of refluxing, the total CO₂ concentrations in the glycineand sarcosine sorbents dropped to 0.42 and 0.32 M, respectively, whichare comparable with the corresponding values observed in theregenerations using PyBIG. With longer reaction times the regenerationunder reflux outperforms the regeneration with PyBIG, with the total CO₂concentrations in solution dropping to 0.26 and 0.17 M after 4 hours ofrefluxing the glycine and sarcosine solutions, respectively. However,such long refluxing times are expected to come at a cost in terms ofenergy consumption and sorbent degradation.

Carbon Dioxide Release and Regeneration of the PyBIG Compound withConcentrated Solar Power

This preliminary study indicated that the PyBIGH₂(CO₃)(H₂O)₄ crystalsrelease the CO₂ and water vapor upon mild heating at temperatures of80-120° C., and regenerate the PyBIG compound quantitatively. However,considering this transformation is highly endothermic, the possibilityof using concentrated solar power to render the process more energysustainable was explored. For the initial small-scale proof of concept,a solar oven (FIG. 6) was used to heat the PyBIGH₂(CO₃)(H₂O)₄ samples(38 mg, 0.1 mmol) at four different temperatures ranging between 120 and160° C., monitoring the extent of the reactions by the samples' weightloss (Table 7 below). FTIR analyses also corroborated the release ofCO₂, most evidently noticeable by the disappearance of the strong peakat 1361 cm⁻¹ corresponding to the stretching mode of the carbonateanion. Thus, these results demonstrate that concentrated solar power caneffectively release the CO₂ from the carbonate crystals and regeneratethe PyBIG compound quantitatively.

TABLE 7 CO2 release from PyBIGH₂(CO₃)(H₂O)₄ and regeneration of PyBIG byheating with concentrated solar power. All measurements were done with38 mg (0.1 mmol) samples. The theoretical weight loss for a quantitativeyield is 13.4 mg (35.1%), corresponding to 0.1 mmol CO₂ and 0.5 mmolH₂O. Experimental uncertainties: T = ±2° C.; Δm = ±1 mg (±2.6%) T (° C.)Heating time (min) Δm (mg) Δm/m_(i) (%) 120 30 −12 −31.6 140 10 −13−34.2 150 5 −12 −31.6 160 2 −14 −36.8

CONCLUSIONS

This study demonstrated a small-scale DAC system using simple,off-the-shelf equipment and readily available chemicals. Effective CO₂absorption with aqueous glycine and sarcosine sorbents using a householdhumidifier was followed by room temperature crystallization of aguanidinium carbonate salt of very low aqueous solubility, and CO₂desorption from the carbonate crystals using concentrated solar power.This approach combines the benefits of an aqueous sorbent, such as fastCO₂ absorption rates, easy handling, and low maintenance, with theadvantages of solid-state CO₂ desorption that avoids much of the energypenalty associated with heating and evaporating aqueous solutions, andminimizes sorbent degradation. Furthermore, the amino acid sorbentsoffer an environmentally friendly alternative to the more traditionalsorbents, such as amines or NaOH. In addition to demonstrating theinitial proof-of-concept, this study also identified a number oflimitations for the current DAC system and provided guidelines for thedesign and optimization of future DAC technologies. First, while theamino acid sorbents react fast with CO₂, the air humidifier used in thisstudy is not optimized for DAC as it provides a relatively smallair-water interfacial area, which limits the overall CO₂ uptake rate.Also, as designed, the humidifier evaporates large amounts of water,which in the case of DAC is a disadvantage. Combining the amino acidsorbents with better air-liquid contactors that optimize the air-waterinterfacial area and minimize the water loss may lead to more efficientDAC systems. Second, while sorbent regeneration and carbonatecrystallization with PyBIG is adequate, with observed cyclic capacitiesof 0.3-0.35 mol/mol, the regeneration process could be significantlyimproved by replacing the PyBIG compound with a more soluble and morealkaline (higher pKa for the guanidine groups) analogue that would pushthe equilibrium for the regeneration reaction further to the right,according to equation 11. Finally, although the CO₂ desorption fromcrystalline PyBIGH₂(CO₃)(H₂O)₄ avoids much of the energy penaltyassociated with heating and evaporating aqueous solutions, the enthalpyof desorption of the carbonate crystals is strongly endothermic, mainlydue to the inclusion of water in the crystals. Engineering carbonatecrystals that are anhydrous, or can release the CO₂ at lowertemperatures to avoid water vaporization, may improve the energyefficiency of the DAC process. On the other hand, employing renewableenergy sources, such as concentrated solar power, as demonstrated inthis study, or low-grade waste heat, may alleviate much of this issue.

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, those skilled in the art maymake various changes and modifications which remain within the scope ofthe invention defined by the appended claims.

What is claimed is:
 1. A compound having the following composition:

wherein A is a ring-containing moiety; one or more of the hydrogen atomsin Formula (1) may be replaced with one or more methyl groups; andwherein Formula (1) optionally further comprises salt forms of thestructure of Formula (1).
 2. The compound of claim 1, wherein thecompound of Formula (1) is a salt form having the following composition:

wherein: A is a ring-containing moiety; X^(m−) is an anionic specieswith a magnitude of charge m, where m is an integer of at least 1; and nis an integer of at least 1; provided that n×m=2.
 3. The compound ofclaim 1, wherein A is a monocyclic ring.
 4. The compound of claim 3,wherein said monocyclic ring is a five-membered, six-membered, orseven-membered ring.
 5. The compound of claim 1, wherein A is acarbocyclic ring or ring system.
 6. The compound of claim 5, whereinsaid carbocyclic ring or ring system is unsaturated.
 7. The compound ofclaim 6, wherein A comprises a benzene ring.
 8. The compound of claim 1,wherein A is a heterocyclic ring or ring system.
 9. The compound ofclaim 8, wherein said heterocyclic ring or ring system is unsaturated.10. The compound of claim 8, wherein said heterocyclic ring or ringsystem contains at least one nitrogen ring atom.
 11. The compound ofclaim 10, wherein A comprises a pyridine ring.
 12. The compound of claim2, wherein X^(m−) is a halide or nitrate.
 13. The compound of claim 2,wherein X^(m−) is sulfate.
 14. The compound of claim 2, wherein X^(m−)is carbonate or bicarbonate.
 15. A method for removing an oxyanion froman aqueous source containing said oxyanion, the method comprising: (i)dissolving an oxyanion precipitating compound into said aqueous sourceto result in precipitation of an oxyanion salt of said oxyanionprecipitating compound; and (ii) removing said oxyanion salt from saidwater containing the oxyanion to result in water substantially reducedin concentration of said oxyanion; wherein said oxyanion precipitatingcompound has the following composition:

wherein: A is a ring-containing moiety; one or more of the hydrogenatoms in Formula (1) may be replaced with one or more methyl groups;X^(m−) is an anionic species with a magnitude of charge m, where m is aninteger of at least 1, provided that X^(m−) is an anionic speciesexchangeable with the oxyanion in said aqueous source before saidoxyanion precipitating compound contacts said oxyanion in step (i), andX^(m−) is said oxyanion in the oxyanion salt of said oxyanionprecipitating compound; and n is an integer of at least 1; provided thatn×m=2.
 16. The method of claim 15, wherein said oxyanion is selectedfrom the group consisting of sulfate, nitrate, chromate, selenate,phosphate, arsenate, carbonate, bicarbonate, and perchlorate.
 17. Themethod of claim 15, wherein said oxyanion is sulfate.
 18. The method ofclaim 15, wherein A is a monocyclic ring.
 19. The method of claim 15,wherein said monocyclic ring is a five-membered, six-membered, orseven-membered ring.
 20. The method of claim 15, wherein A is acarbocyclic ring or ring system.
 21. The method of claim 20, whereinsaid carbocyclic ring or ring system is unsaturated.
 22. The method ofclaim 21, wherein A comprises a benzene ring.
 23. The method of claim15, wherein A is a heterocyclic ring or ring system.
 24. The method ofclaim 23, wherein said heterocyclic ring or ring system is unsaturated.25. The method of claim 23, wherein said heterocyclic ring or ringsystem contains at least one nitrogen ring atom.
 26. The method of claim25, wherein A comprises a pyridine ring.
 27. A method for removingcarbon dioxide from a gaseous source, the method comprising: (i)contacting said gaseous source with an aqueous solution containing acarbon dioxide complexing compound to result in precipitation of acarbonate or bicarbonate salt of said carbon dioxide complexingcompound; and (ii) removing said carbonate or bicarbonate salt from saidaqueous solution; wherein said carbon dioxide complexing compound hasthe following composition:

wherein: A is a ring-containing moiety; one or more of the hydrogenatoms in Formula (1) may be replaced with one or more methyl groups;X^(m−) is an anionic species with a magnitude of charge m, where m is aninteger of at least 1, provided that X^(m−) is hydroxide in said carbondioxide complexing compound before said carbon dioxide complexingcompound is contacted with carbon dioxide in step (i), and X^(m−) iscarbonate or bicarbonate in said carbonate or bicarbonate salt; and n isan integer of at least 1; provided that n×m=2.
 28. The method of claim27, wherein A is a monocyclic ring.
 29. The method of claim 28, whereinsaid monocyclic ring is a five-membered, six-membered, or seven-memberedring.
 30. The method of claim 27, wherein A is a carbocyclic ring orring system.
 31. The method of claim 30, wherein said carbocyclic ringor ring system is unsaturated.
 32. The method of claim 31, wherein Acomprises a benzene ring.
 33. The method of claim 27, wherein A is aheterocyclic ring or ring system.
 34. The method of claim 33, whereinsaid heterocyclic ring or ring system is unsaturated.
 35. The method ofclaim 33, wherein said heterocyclic ring or ring system contains atleast one nitrogen ring atom.
 36. The method of claim 33, wherein Acomprises a pyridine ring.